Could Black Trees Blossom in a World With Two Suns?

Press release from The UK National Astronomy Meeting - 19th April 2011

A sky with two suns is a favourite image for science fiction films, but how would a binary star system affect life evolving on an orbiting planet? Jack O’Malley-James of the University of St Andrews has studied what plants might be like on an Earth-like planet with two or three suns and found that they may appear black or grey. He will be presenting results at the RAS National Astronomy Meeting in Llandudno on Tuesday 19th April.

In M star radiation habitats, vegetation may have more photosynthetic pigments in order to make use of a fuller range of wavelengths, giving them a ‘black’ appearance. Above: Terrestrial examples of a dark plants (and flowers).

Photosynthesis - converting sunlight into energy - is the basis for the majority of life on Earth. It is the energy source for plants and, hence, animals higher up the food chain. With multiple light sources, life may have adapted to use all suns, or different forms may develop that choose to use one specific sun. This may be the more likely option for planets on which parts of the surface are illuminated by only one sun for long periods of time.

“If a planet were found in a system with two or more stars, there would potentially be multiple sources of energy available to drive photosynthesis. The temperature of a star determines its colour and, hence, the colour of light used for photosynthesis. Depending on the colours of their star-light, plants would evolve very differently,” said O’Malley James.

O’Malley-James is working on a PhD (funded by the STFC Aurora scheme) supervised by Dr Jane Greaves at the University of St Andrews, Prof John Raven (University of Dundee) and Prof Charles Cockell (The Open University) to assess the potential for photosynthetic life in multi-star systems with different combinations of Sun-like stars and red dwarfs. Sun-like stars are known to host exoplanets and red dwarfs are the most common type of star in our Galaxy, often found in multi-star systems, and old and stable enough for life to have evolved. Over 50% of Sun-like stars and 25% of red dwarfs are found in multi-star systems. In the team’s simulations, the Earth-like planets either orbit two stars close together or orbit one of two widely separated stars. The team has also looked at combinations of these scenarios, with two close stars and one more distant star.

Gliese 667 is one of two multiple star systems (the other being 55 CnC) known to host planets below 10 Earth masses. Image Credit: ESO/L. Calçada

“Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth. Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis. For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sun-screens, or photosynthesising microorganisms that can move in response to a sudden flare,” said O’Malley-James.

The last survivors at the end of the world

Press release from The UK National Astronomy Meeting - July 2013

In 2 billion years’ time, life on Earth will be confined to pockets of liquid water deep underground, according to PhD astrobiologist Jack O’Malley James of the University of St Andrews. The new research also suggests that though the hardiest forms of life may have a foothold on similar worlds in orbit around other stars, evidence for it may be very subtle. O’ Malley- James will present the findings at the National Astronomy Meeting in St Andrews, Scotland.

An image of the Upper Geyser Basin region in Yellowstone National Park in Wyoming, USA. As the Sun heats up, much of the Earth will come to resemble this landscape. Credit: Jack O’Malley-James.

All species have finite lifetimes, with each eventually facing an event that leads to its extinction. This can be sudden and catastrophic, like the giant impact that wiped out the dinosaurs, or a slow and gradual process. Ultimately, a combination of slow and rapid environmental changes will result in the extinction of all species on Earth, with the last inhabitants disappearing within 2.8 billion years from now.
The main driver for these changes will be the Sun. As it ages over the next few billion years, the Sun will remain stable but become steadily more luminous, increasing the intensity of its heat felt on Earth and warming the planet to such an extent that the oceans evaporate. In his new work, O’Malley James has created a computer model to simulate these extremely long-range temperature forecasts and has used the results to predict the timeline of future extinctions.
Within the next billion years, increased evaporation rates and chemical reactions with rainwater will draw more and more carbon dioxide from the Earth’s atmosphere. The falling levels of CO2 will lead to the disappearance of plants and animals and our home planet will become a world of microbes. At the same time the Earth will be depleted of oxygen and will be drying out as the rising temperatures lead to the evaporation of the oceans. A billion years after that the oceans will have gone completely.

An electron microscope image of thermophilic (heat-loving) bacteria. These organisms may be amongst the last life on Earth, perhaps surviving 2.8 billion years into the future. Credit: Mark Amend / NOAA Photo Library.

"The far-future Earth will be very hostile to life by this point", said O'Malley-James. "All living things require liquid water, so any remaining life will be restricted to pockets of liquid water, perhaps at cooler, higher altitudes or in caves or underground". This life will need to cope with many extremes like high temperatures and intense ultraviolet radiation and only a few microbial species known on Earth today could cope with this.
The new model not only tells us a lot about our own planet's future, but it can also help us to recognise other inhabited planets that may be approaching the end of their habitable lifetimes. O’Malley-James adds "When we think about what to look for in the search for life beyond Earth our thoughts are largely constrained by life as we know it today, which leaves behind telltale fingerprints in our atmosphere like oxygen and ozone. Life in the Earth's far future will be very different to this, which means, to detect life like this on other planets we need to search for a whole new set of clues".
"We have now simulated a dying biosphere composed of populations of the species that are most likely to survive to determine what types of gases they would release to the atmosphere. By the point at which all life disappears from the planet, we're left with a nitrogen:carbon-dioxide atmosphere with methane being the only sign of active life".